The corrosion of metal equipment is a widespread and expensive problem in many industrial settings such as open recirculating cooling towers and in the agricultural industry in fertilizer storage and irrigation systems. Expenses arise due to corrosion from lost production time and from costly repairs and replacement of metal equipment parts that have been weakened or destroyed by the action of water and oxygen at the metal surface.
Many environmental factors can accelerate the corrosion rate of metals, and ferrous metals in particular, in aqueous systems. For example, a high content of dissolved oxygen in an aqueous system has a significant oxidative influence on the corrosion rate of many metals. Inert gas purging or blanketing of aqueous systems can minimize the oxygen content of the aqueous solution to decrease the rate of corrosion, but this approach is limited to closed systems. Another factor is the temperature of the aqueous system. A reduction in the temperature of the aqueous system can decrease the rate of corrosion providing the reduced temperature can be tolerated and controlled. Similarly, a reduction in the velocity or movement of the aqueous system through the equipment can also reduce the rate of corrosion. However, some flow is essential to maintain corrosion protection when utilizing corrosion inhibitors. Finally, maintaining a pH value of about 7 units or greater in the aqueous system can lessen the corrosion rate of metals caused by acid attack.
Another important means of retarding corrosion in aqueous systems is the use of various chemical additives as "corrosion inhibitors." It is believed that these additives inhibit the rate of corrosion by forming surface films which physically block the diffusion of ions to and from the surface of the metal. Some of these corrosion inhibitors are organic polymers, others are salts of toxic metals such as chromium, still others are phosphorous-containing complexes. See, for example, Little et. al., "Corrosion Inhibition by Thermal Polyaspartate," Surface Reactive Peptides and Polymers: Discovery and Commercialization, Sikes, C. S. and Wheeler, A. P. (Eds.), ACS Symposium Series No. 444 (1990). However, many of these chemical additives have a negative environmental impact given their toxicity and persistence.
In industrial water treatment applications, such as open recirculating cooling systems, and boilers, the formation of scale and corrosion is a major problem. The growth of mineral crystals (scale formation) on heat transfer surfaces leads to decreased heat transfer efficiency and increases corrosion of heat transfer components. Corrosion of the heat transfer components can lead to costly replacements and failures. The addition of chemicals to reduce scale is well known. In many cases, phosphorus-containing materials are used to inhibit scale. However, when released into the environment, many of these materials can have a negative environmental impact. Polymeric organic inhibitors of scale are also known, such as polyacrylic acid, polymaleic acid, copolymers of acrylic and maleic acid, copolymers of acrylic acid and acrylamide, and the like. These polymeric materials, while being good scale inhibitors, are not known to have corrosion inhibition activity. Moreover, these materials are also not readily biodegradable, and thus can contribute to the overall chemical burden on the environment for long periods of time.
The performance of polymers as mineral scale inhibitors is dependent on the ability of the polymers to interact with growth sites on the surface of growing mineral crystals to retard mineral formation and reduce the adherence of the mineral to heat transfer surfaces by changing the crystal morphology.
Therefore, a need exists for an effective and environmentally tolerable polymeric corrosion inhibitor that can be used to inhibit corrosion and mineral scale on metal in contact with a metal-corrosive aqueous systems. Some previous attempts to fulfill this need employed a solution containing aspartic acid homopolymers to inhibit the corrosion of metals in aqueous systems of basic pH (U.S. Pat. No. 4,971,724 to Kalota et al.); an aqueous detergent solution combined with homopolymers of aspartic acid or glutamic acid (U.S. Pat. No. 5,531,934 to Freeman et al.); and polyaspartate salts having alkali or alkaline earth metal counterions as corrosion inhibitors in an aqueous saline environment (U.S. Pat. No. 5,607,623 to Benton et al.). However, corrosion inhibition has met with limited success.
The present invention provides for both scale and corrosion inhibition benefits in a single, substantially non-toxic, environmentally acceptable polymer-containing composition.